Internal combustion engines on the basis of gasoline engines are generally operated with fuel from hydrocarbons produced from fossil fuels based on refined crude oil. Ethanol produced from renewable resources (plants) or another kind of alcohol is increasingly being added in various mixing ratios to the fuel. In the USA and Europe a mixture of 75-85% ethanol and 15-25% gasoline is often distributed under the trade name E85. The internal combustion engines are designed in such a way that they can be operated with pure gasoline as well as with mixtures up to E85 or even pure ethanol (E100). This is denoted as a “flex-fuel operation”. The operating parameters in the flex-fuel operation have to be adapted in each case to the existing fuel mixture for an efficient operation with only a small discharge of toxic emissions; while at the same time high engine performance is guaranteed. A stoichiometric fuel-air mixture ratio is, for example, present at 14.7 mass parts of air per part of gasoline; however, when using pure ethanol, a proportion of air of 9 mass parts must be set.
The taking into account of the fuel, which has ingressed into the engine oil during cold starting, and the fuel evaporating out of the engine oil at high temperatures has to fundamentally take place with all internal combustion engines; however, is mainly employed with gasoline engines with a flex-fuel-mode of operation.
Ethanol and gasoline have different evaporation properties. Ethanol evaporates at a lower temperature to a lesser degree than gasoline, so that more ethanol has to be injected during cold starting than is the case for gasoline. When low ambient temperatures prevail in the winter months, this effect is again increased. This leads to the fact that when cold starting with ethanol and during the subsequent warm-up phase, considerably more fuel is wiped from the cylinder walls into the engine oil by the piston rings. When the temperature of the engine oil increases, this fuel evaporates and is added via the crankcase ventilation system to the fresh air supply provided to the engine by way of the intake manifold. The total amount of fuel situated in the engine oil is only expelled during extended warm-up phases. The additional richening of the mixture caused by the evaporated fuel has to be taken into account. Especially during low load-rotational speed-conditions, this richening can be significant. During idling with the engine operating with gasoline, the richening of the mixture from the outgassing of fuel can definitely reach 30%. At a low temperature, ethanol evaporates out of the oil to a greater extent than gasoline. For this reason, the richening of the fuel mixture by the outgassing of fuel can even be higher in this instance during a flex-fuel mode of operation. The richening of the fuel mixture can be taken into account by the closed-loop lambda control at certain percentages. This can, however, be insufficient in certain cases and lead to a degradation of the exhaust gas emission values with undesirably high values of hydrocarbons and carbon monoxide on account of the overall mixture being too rich.
It is the task of the invention to provide a method, which allows for an improved way of taking into account the outgassing of fuel from the engine oil of an internal combustion engine and which also brings about the combustion of the fuel-air mixture with lower exhaust gas emissions when the outgassing of fuel from the engine oil is high.